When Less Is More: How A Class X Master Can Outperform A Class XXX

This may sound like the wrong title for a gaging column. You’d think I’d be saying something like, “when more is better,” and talking about how gage performance is improved when the gage is larger and has more mass.

Columns
From:
5/19/2009
Modern Machine Shop

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George Schuetz

Let’s use bench stands as an example. Gage stability is based on mass, so a large bench stand with a heavy base and a substantial post and arm is going to be much more stable than its lightweight cousin. Having a solid, more stable gage means less deflection, slower thermal reaction and less measuring error. That makes perfect sense.

However, in the case of using hard masters (rings and discs), less can be more. That is, you can actually improve your measuring process by purchasing a lower-grade master. A lower-class master that is certified to size costs less than its higher-class cousin, and it will provide a more accurate reading when used with its certified master deviation.

Master ring and plug gages are made from hardened steel; chromed steel for durability and corrosion resistance; or tungsten carbide for extreme wear resistance. They are categorized into classes by level of accuracy, with XXX indicating the tightest tolerances. XX, X and Y are intermediate grades (in descending order), and Z is the lowest level of accuracy. Class tolerances vary by size: Larger sizes have higher levels of uncertainty. The table on page 60 provides an example of how the tolerance tables work for the various classes of master rings or discs.

Traditionally, the rule of thumb for ordering a master ring for a gage requirement has been pretty straightforward: buy the best you can afford. Until recently, there was no consideration for the actual tolerance, because it was assumed that the best class master would ensure the best measurement result.

However, things have changed in the world of gaging, calibration and traceability. To meet today’s ISO and other standards, customers now require documentation about the measurement process on the parts they are buying. Now, even if manufacturers buy the best-class master, they still need a document that says the master is certified to a particular class. Alternately, they need a document that actually certifies the exact size of the master to show that the master was within the tolerances specified for the class of the master.

Here’s where less can be more. Let’s take an example with a 1-inch steel master ring. If you order a XXX master ring from a supplier and have it certified for either class or size, you will pay at least double the price of the same 1-inch steel master ring, class X. Not only will you pay less money for a lower-class master, but you will also spend less time waiting to get delivery from the vendor. Also, the XXX ring, if used regularly, will be apt to go out of tolerance within a few years anyway.

The better measurement comes from using the information you paid for in the certificate you received. Remember, that certificate gives you the exact size of the master.

The saying is that “information is power,” and this is especially true with certified masters. Let’s look at the same 1-inch master ring you had certified to class. It has a potential error of ±15 microinches. When you put this master on the gage and “zero it out,” you are not taking into account this error.

On the other hand, the 1-inch class X ring has a tolerance range, or potential error, of ±60 microinches, but with it you also get a document that shows it is off by some specific amount, say +55 microinches. Now, you can actually “dial in” this master deviation, and when zeroed out, the gage displays 55 microinches instead of “0.” What is really happening is that the gage setting with the X master is more precise than the gage set with the XXX master.

Now, let’s look at the fine print. Most gages don’t have resolutions to 1 microinch but many do resolve to 10 microinches, including gages such as air tooling, bench amplifiers and even some good digital and dial comparators. If you can squeeze an extra 40 microinches of accuracy out of a gage that is measuring a ±0.0001-inch tolerance (the difference between the ±15 microinch class XXX tolerance and the +55 microinch gage setting), you have just improved the process by 20 percent. Not a bad improvement while paying 50 percent less for the master.

The point here is that when you have the capability of displaying to a high resolution and you know the actual size of the master, you can get more accuracy out of your measurement process. By using a master that is less accurate—but knowing the size and using it in your mastering routine—you can get better measurement results: more for less.

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